Telescopes are increasingly being launched into space and operated outside of the Earth's atmosphere. The reason for this is that by positioning a telescope in space, the distorting effect of the Earth's atmosphere on light that is collected from distant objects can be reduced or eliminated altogether. As with a telescope that is mounted on the surface of the Earth, the resolution that is provided by a telescope that is mounted in space is directly related to the size of the primary mirror/collector (referred to herein as “an aperture”) that is used to collect light from the distant object. The larger the aperture, the finer the resolution of the image that is obtained of the distant object.
This relationship between the size of the aperture and the resolution of the image creates a motivation to have the aperture of a space telescope be as large as possible. For example, it may be desirable to have an aperture that has a diameter that extends for dozens or hundreds of meters or more. A limiting factor on the size of the aperture of a space telescope is the current lift capability of modern spacecraft. It is not presently possible to launch an object into space having a diameter of dozens or hundreds of meters because modern spacecraft cannot accommodate a payload of that size.
One solution to this problem has been to design a space telescope having multiple apertures (referred to herein as “sub-apertures”) that are positioned with respect to one another so as to behave like a single aperture. The sub-apertures can be connected to one another by structures and machinery that control the movement of each sub-aperture. Together, the sub-apertures, the structures that connect them to one another, and the machinery that controls their movement will be referred to herein as a multiple-aperture imaging system. There are many designs for multiple-aperture imaging systems, including segmented primary mirrors and multiple-telescope arrays. In many cases, a multiple-aperture imaging system can be reconfigured such that the sub-apertures may be arranged to form a condensed package that is compatible with the payload constraints of modern spacecraft. In this manner, a multiple-aperture imaging system that has a relatively large aperture can, nevertheless, be launched into space using a modern spacecraft.
Once the multiple-aperture imaging system arrives in space, it must be unpacked from the payload compartment of the spacecraft, unfolded, and then placed into an operational configuration with each sub-aperture positioned in its respective operational location. In order to function optimally, the sub-apertures must be very closely aligned with one another. Preferably, they will be positioned such that the reflective surfaces of the respective sub-apertures behave like a single aperture. In most cases, this requires that the reflective surfaces of the sub-apertures be aligned to within a small fraction of a reference wavelength of light.
Fine-alignment processes, such as phase retrieval or phase diversity, that permit the sub-apertures of a multiple-aperture imaging system to be aligned to within a small fraction of a reference wavelength of light are well known in the art. One such process, known as phase diversity, is described in “Joint estimation of object and aberrations by using phase diversity,” Journal of the Optical Society A, Volume 9, July 1992, pages 1072-1085, by Paxman, et al. Phase diversity works very well to bring relatively closely aligned sub-apertures into very close alignment (i.e., within a small fraction of a reference wavelength of light), but it does not work well to bring sub-apertures from a highly unaligned state into very close alignment. Instead, other processes are utilized to move unaligned sub-apertures into a state of relatively close alignment (referred to as coarse alignment) before the phase-diversity process, or other such fine-alignment processes, can be implemented. These other coarse-alignment processes may entail sequentially aligning multiple pairs of sub-apertures in an iterative manner. Such methods are generally slow and time consuming It is desirable to bring the unaligned sub-apertures of a multiple-aperture imaging system into a state of relatively close alignment more quickly and/or more efficiently than current methods permit. Alternative coarse-alignment processes may require the use of additional hardware such as magnetic or interferometric sensors that operate between subapertures. It is desirable to reduce the number of such inter-subaperture sensors as well as the associated hardware complexity, weight, and cost. Alternative coarse-alignment processes may also have limits to the range of piston misalignments that they can sense. It is desirable to increase the range of piston misalignments that can be accommodated in coarse alignment.